50 
The Light from the Stars [oh. ii 
These absorption-lines are among the most informative objects in the 
whole of astronomy. In accordance with Dopplers principle, a difference of 
velocity v between a source emitting light and an instrument receiving the 
light, causes the wave-length A/ of the light received, to be different from 
the wave-length \ of the emitted light, the two being connected by the 
relation V = \ ^1 + , where G is the velocity of light. Consequently the 
motion of a star causes the lines in its spectrum to shew a slight displacement 
from their standard positions, and by measuring the amount of this displace 
ment it is possible to determine the star’s velocity of recession or approach, 
relative to the moving earth. We have already noticed how W. S. Adams and 
Kohlschiitter found that certain peculiarities in these lines made it possible 
to determine the absolute magnitude of the star by which they are emitted, 
and hence its distance. Further the Einstein theory of relativity requires that 
each spectral line should shew a displacement towards the red end of amount 
proportional to Mir, the gravitational potential at the surface of the emitting 
star, so that if the displacement can be measured in the light emitted by 
a particular star, the value of M/r is at once known. This method has recently 
been used to determine the value of M/r for the companion of Sirius. The 
mass M of this star was already known, from its gravitational pull on Sirius, 
to be about 0‘85 times the mass of the sun, so that it became possible to 
calculate the star’s radius r. This proved to be only about 20,000 kms., 
shewing that the mean density of the star must be about 50,000 times that 
of water. 
Finally, a comparison of the positions of the absorption bands of a stellar 
spectrum with those obtained from known chemical substances in the 
laboratory, makes it possible to identify the atoms or molecules which absorb 
the light in the star’s surface. Practically all of the lines in the solar spectrum 
have been identified in this way, and are found to originate from the atoms of 
elements known on earth; a large number of the lines originating from the 
upper layers of the sun’s atmosphere have their origin in ionised atoms, 
particularly those of calcium, strontium and barium. 
Table VII (opposite)* gives the 90 chemical elements which are known on 
earth, arranged in order of their atomic number, with an indication of 
whether they are represented (P) or absent (A) in the spectrum of the sun. 
Sahaf and, more recently, R. H. Fowler and Milne J, have shewn how 
this identification of absorption lines in stellar spectra makes it possible to 
estimate the temperature of the absorbing atoms, and hence the effective 
* Compiled from Bussell, Dugan and Stewart, Astronomy, p. 503; Miss Payne, Stellar Atmo 
spheres, pp. 5 and 184; Publicat. Ast. Soc. Pac. 39 (1927), p. 238; A'strophys. Journ. lxviii. (1928), 
p. 327, etc. 
f Phil. Mag. xn. (1920), pp. 472, 809. Proc. R.S. 99a (1921), p. 135. Zeit.f. Phys. vi. (1921), 
p. 40. 
M.N., R.A.S. lxxxiii. (1923), p. 403.